In a prospective study of maternal serum alpha-fetoprotein screening for both high and low values, we assessed the overall predictive value, sensitivity, specificity and relative risks for congenital defects and complications of pregnancy. Among 13,486 women with singleton pregnancies interviewed at the time of screening (15 to 20 weeks of gestation), 3.9% had high and 3.4% had low values. A high maternal serum alpha-fetoprotein value was associated with the following adverse outcomes: neural tube defects (relative risk = 224), other major congenital defects (relative risk = 4.7), fetal deaths (relative risk = 8.1), neonatal death (relative risk = 4.7), low birth weight (relative risk = 4.0), newborn complications (relative risk = 3.6), oligohydramnios (relative risk = 3.4), abruptio placentae (relative risk = 3.0) and preeclamptic toxemia (relative risk = 2.3). A low maternal serum alpha-fetoprotein value was associated with chromosomal defects (relative risk = 11.6) for fetal death (relative risk = 3.3). Either high or low maternal serum alpha-fetoprotein values were associated with 34.2% of all major congenital defects, 19.1% of all stillbirths and fetal-neonatal deaths, 11.0% of major pregnancy complications, and 15.9% of serious newborn complications. Maternal serum alpha-fetoprotein screening provides an important adjunctive tool for the identification of high-risk pregnancy and adverse neonatal outcome.